1. Field of the Invention
This invention relates, generally, to the utilization of waste gas streams containing chlorine and carbon dioxide and, particularly, to waste gas streams produced during the production of chlorine dioxide by the reaction of an alkali or alkaline earth metal chlorate with a reducing agent.
2. Description of the Prior Art
Chlorine dioxide is an oxidation agent and an important bleaching agent in the pulp industry where it is the most common bleaching agent used in the final stages of pulp bleaching. Recently, there has been an increased use of chlorine dioxide instead of other bleaching agents especially sodium hypochlorite and chlorine. It is known that the use of chlorine in bleaching pulp, lead to the production of dioxins which are released to the environment in the disposed wastes. Sodium hypochlorite solutions, when used in bleaching pulp, lead to the formation of chloroform which cannot be tolerated in the paper industry at any concentration. Accordingly, there is a decreased use of chlorine and sodium hypochlorite as bleaching agents in the wood pulp industry.
Methods formerly used for the preparation of chlorine dioxide by reduction of sodium chlorate can be summarized in the following gross formulas: ##STR1##
Thus, the reducing agent in these processes is sulphur dioxide, methanol and chloride ion respectively. Other reducing agents, such as chromic acid or nitrogen oxides have also been tested, but, principally, due to their higher prices they have not been commercially utilized to a considerable degree.
A modern method for chlorine dioxide production is as follows: ##STR2##
All these processes take place with an excess of a strong acid, usually sulfuric acid. In process for the production of chlorine dioxide in which sulfuric acid is used, the spent liquor or saltcake of the reactor will consist of sodium sulphate and/or strong sulfuric acid or, if desired, sodium hydrogen sulphate in strong sulfuric acid. The gases carbon dioxide and possibly some chlorine are produced in the Rapson R-8 process. Passage of these gases through a sodium hydroxide scrubber produces an aqueous liquor stream of sodium chloride, sodium hypochlorite and sodium carbonate. It is essential from an economical as well as an environmental point of view that this liquor be utilized. However, this waste stream cannot be used because of the generation of chloroform from the sodium hypochlorite formed during the scrubbing of chlorine with sodium hydroxide in the wood pulp bleaching process.
The process of reacting chlorine with caustic to made sodium chlorate has been known for over 100 years. Under alkaline conditions chlorine reacts with caustic to make sodium hypochlorite: EQU Cl.sub.2 +2NaOH.fwdarw.NaClO+NaCl+H.sub.2 O V.
Under approximately neutral conditions, sodium hypochlorite reaches an equilibrium with hypochlorous acid and forms sodium chlorate: EQU NaClO+2HClO.fwdarw.NaClO.sub.3 +2HCl VI.
Making sodium chlorate this way is generally not practiced because it is cheaper to make sodium chlorate by the direct electrolysis of salt: EQU NaCl+3H.sub.2 O+6 Faradays.fwdarw.NaClO.sub.3 +3H.sub.2 VII.
Presently, caustic is used to remove chlorine from tail gas streams from sodium chlorate, chlorine or chlorine dioxide plants according to reaction V. The resulting sodium hypochlorite solution has typically been a useful chemical in such applications as pulp bleaching, and the pulp and paper industry has always provided a useful market for this chemical.
Sodium hypochlorite is being phased out of wood pulp bleaching because it forms chloroform on the pulp. This poses a particularly severe problem in the generation of chlorine dioxide because, whereas tail gases from sodium chlorate and chlorine plants contain only trace amounts of chlorine, chlorine dioxide generators of the R2, R3, and SVP type produce 0.6-0.7 lb of chlorine per lb of chlorine dioxide and frequently the concentration of chlorine in the tail gases is over 70%. Further contributing to the problem is the trend in the pulp and paper industry to increase chlorine dioxide production because chlorine dioxide has been shown to reduce formation of dioxin and chlorinated organics when substituted for chlorine in the pulp bleaching process.
Any NaCl which enters a chlorine dioxide generator results in an equivalent amount of Cl.sub.2 by-product. To address this concern pulp and paper mills are switching to a new type of chlorine dioxide generator which uses methanol as the reducing agent instead of NaCl, thereby producing CO.sub.2 as a by-product rather than Cl.sub.2. However some by-product chlorine can still be produced from generator inefficiency or decomposition, or from any salt present in the sodium chlorate feed. For example, sodium chlorate from solution sodium chlorate plants would contain 0.1-0.3 moles of NaCl per mole of NaClO.sub.3. Also, although claims are made that the use of methanol as a reducing agent in chlorine dioxide generators allows the omission of chloride ion in the sodium chlorate feed, it is well documented that their operation is more stable if some chloride ion is present.
Chlorine resulting from any of these reasons can end up in the chlorine dioxide generator chlorine tail gas scrubber where it will be removed according to reaction V. However, by-product CO.sub.2 from the methanol will also be scrubbed according to reaction VIII below thus creating a scrubber effluent which is no longer a simple sodium hypochlorite solution, but rather a mixture of sodium hypochlorite and sodium carbonate. EQU CO.sub.2 +2NaOH.fwdarw.Na.sub.2 CO.sub.3 +H.sub.2 O VIII.
Since these sodium hypochlorite solutions are no longer desirable pulp bleach chemicals, they have to be dealt with in other ways. An obvious approach is to neutralize the solutions with acid so the sodium hypochlorite will convert to sodium chlorate according to reaction VI, and then reprocess the resulting sodium chlorate solution in the chlorine dioxide generators or in any available sodium chlorate plant. However the presence of sodium carbonate in the solutions makes this approach impractical by any conventional means because the neutralization process will drive off the sodium carbonate as CO.sub.2 which causes several problems which render prior art processes unusable:
(a) The liberated CO.sub.2 will contain high levels of chlorine and/or sodium hypochlorite and will therefore have to be scrubbed, but any conventional scrubber will also remove the CO.sub.2 as sodium carbonate thereby creating a closed system where sodium carbonate will build up to the saturation point and eventually shut down the process. PA1 (b) The tiny bubbles of CO.sub.2 forming in the solution cause severe foaming which renders most conventional reactors unusable. PA1 (c) The sodium carbonates will consume expensive acid and yield only low value sodium chloride salt according to reaction IX. below: EQU Na.sub.2 CO.sub.3 +2HCl.fwdarw.CO.sub.2 +2NaCl+H.sub.2 O IX. PA1 1) The chlorine is scrubbed from the effluent gases with an aqueous sodium hypochlorite scrubber solution. PA1 2) The heat in the effluent gases emanating from the process reactors is used to preheat said scrubber solution while the sodium hypochlorite scrubber solution cools and condenses water vapor and sodium hypochlorite from the effluent gases. PA1 3) The last traces of chlorine from the effluent gases can be removed with an optional reducing agent. PA1 4) Any strong acid can be used as a substitute for said chlorine gas reactant provided that the sodium hypochlorite is converted to sodium chlorate and free alkalinity is converted to sodium chloride or other salts.
In a similar fashion, sodium carbonate could also build up over time from chlorine tail gas scrubbers in sodium chlorate, chlorine or water treatment plants, or any other process where chlorine is used, because of CO.sub.2 coming from the atmosphere, floor washings or from a brine purification area. Therefore a system is needed which will treat any chlorine tail gas or sodium hypochlorite solution in such a way that unwanted CO.sub.2 or sodium carbonate will be separated and purified of entrained chlorine or sodium hypochlorite compounds such that the unwanted CO.sub.2 can be safely discharged from the process, and the sodium hypochlorite and/or any free alkalinity in the effluent will be converted to a usable sodium chlorate solution suitable for reprocessing even in a closed loop system.
Ninety-five percent of the sodium chlorate produced is used in the pulp and paper industry to manufacture chlorine dioxide. Chlorine dioxide has been shown not to produce dioxins and other chlorinated organics when substituted for chlorine in the pulp bleaching process. Accordingly, the use of chlorine for bleaching with pulp has been sharply reduced and the demand for chlorine dioxide in the pulp and paper industry has risen rapidly over the past few years. Since chlorine dioxide is made by reducing an aqueous solution of sodium chlorate, as indicated above, pulp mills have two options for supplying chlorine dioxide generators with an aqueous solution of sodium chlorate: (1) purchase sodium chlorate crystal and obtain shipment thereof via railcar or truck and (2) manufacture an aqueous solution of sodium chlorate on the site of the pulp mill. The advantages of manufacture of an aqueous solution of sodium chlorate at the pulp mill are well documented in the literature. Most of the advantages to be obtained are the result of the ability of pulp mill sodium chlorate manufacturing facilities to prepare the aqueous sodium chlorate solution at the proper concentration and quality desired so as to properly feed the chlorine dioxide generators directly. This procedure eliminates crystallizing the sodium chlorate, shipping, unloading, and handling the sodium chlorate which would be otherwise purchased from off-site manufacturers. With pulp mill sodium chlorate requirements ever increasing, the advantages of on-site sodium chlorate solution manufacture are greater than ever.
Alkali metal chlorate, and in particular sodium chlorate has been produced by the electrolysis of aqueous solutions of alkali metal chlorides, such as sodium chloride, in electrolytic cells equipped with or without membranes or diaphragms. Typically, electrolytic cells make sodium chlorates within the cell by reacting chlorine produced at the anode with sodium hydroxide produced at the cathode. One such representative electrolytic cell of this type is shown in U.S. Pat. No. 3,732,153 by C. J. Harke et al. Various other arrangements of both electrochemical and combinations of electrochemical and chemical methods for manufacturing sodium chlorates have also been proposed, such as the use of a two compartment permselective membrane equipped electrolytic cell operating in conjunction with a diaphragmless-type electrolytic sodium chlorate cell. This method is disclosed in U.S. Pat. No. 3,897,320 to E. H. Cook. However, to obtain improved current efficiencies and significant reductions in electrical power requirements in the production of inorganic sodium chlorate, U.S. Pat. No. 3,464,901 provides for the electrochemical preparation of chlorine and caustic soda in a diaphragm type chloralkali cell. The caustic soda containing unreacted alkali metal chloride and alkali metal chlorate is then removed from the cell and mixed and chemically reacted with chlorine from the anolyte of the cell. The chemical reaction is carried out at a pH of 6 to 8 to convert the sodium hypochlorite to sodium chlorate. However, in order to maintain the conditions most favorable for converting sodium hypochlorite to sodium chlorate, additional caustic and/or acid over and above that supplied by the cell has to be added to the reaction mixture. In the case of Japanese Pat. No.792,025 dilute chlorine is reacted with less than 20 percent caustic soda to produce a concentrated sodium hypochlorite solution with sufficient caustic remaining in it to produce a pH of 8 to 10. The solution is subsequently diluted from about 13 to 15 percent sodium hypochlorite to 6 to 8 percent sodium hypochlorite with a recycled stream of sodium chloride and sodium chlorate. The diluted stream is then acidified with hydrochloric acid to a pH of about 6.0 and finally fed to an electrolysis cell.
In U.S. Pat. No. 4,175,038 to Sakowski, a process is disclosed for reducing the available chlorine content of aqueous waste streams, especially calcium hypochlorite waste streams. In the process of this reference, the available chlorine content is reduced by chlorinating the impure stream at a temperature in the range of about 80.degree. to 100.degree. C. at a pH in the range of about 5.5 to about 8.5. During this reaction, the available chlorine is reacted to form the corresponding calcium chlorate.
In U.S. Pat. No. 4,159,929 to Grotheer, a process is disclosed for producing alkali metal chlorates by the reaction of an aqueous solution of an alkali metal chloride, alkali metal chlorate and an alkali metal hypochlorite with an alkali metal hydroxide. Chlorine is added to the reaction mixture in an amount sufficient to maintain the pH of the reaction mixture at about 5-7.5 in order to promote the conversion of alkali metal hypochlorite to alkali metal chlorate. Subsequently, the reaction product is led to an electrolysis cell for the production of an alkali metal chlorate. Instead of feeding brine to the electrolytic sodium chlorate cells, the feed solution is made, for instance, by reacting a sodium hydroxide solution with chlorine at neutral pH to make a weak sodium chlorate solution which is then electrolyzed in electrochemical cells to a strong sodium chlorate solution. Gaseous chlorine is added to the caustic in an in-line mixer at 70.degree.-80.degree. C. in an amount such that the pH of the mixture is controlled at 5.0-7.5. The resulting sodium hypochlorite solution is then held in an aging tank to allow the sodium hypochlorite to convert to sodium chlorate.
In Grotheer, the chlorine and chemical feeds to the process are relatively pure and no provision is made to deal with situations where sodium carbonates may be present, such as tail gases from a methanol type chlorine dioxide generator. The system does not provide any way to purify or handle effluent gases which may emanate from the chlorine/caustic reaction, nor does it show any way to deal with foam which would accompany such gases. Also, the in-line mixer and aging tank are not vented and any gases emanating from the reaction, such as CO.sub.2, could create unsafe pressures. Venting these vessels would release chlorine and sodium hypochlorite to the atmosphere, but employing a scrubber would return such gases as CO.sub.2 to the system as sodium carbonate or sodium bicarbonate which would build up to the saturation point and shut the process down.
Also, Grotheer does not produce a sodium chlorate solution low enough in sodium hypochlorite concentration to be purified by conventional means, such as ion exchange. This means that dilute sodium hypochlorite solutions, such as would be discharged from a chlorine tail gas scrubber, would not be suitably treated to be, for example, saturated with NaCl and purified for recycle to electrolytic sodium chlorate cells.
In summary, the described process of Grotheer is not able to handle chlorine tail gases or sodium hypochlorite solutions which contain CO.sub.2 or sodium carbonate, nor is the process able to produce a sodium chlorate solution which can be recycled in a closed loop system unless the component chemicals are predictably very, pure.
In U.S. 4,216,195 to Jaszka, the production of chlorine dioxide having a low chlorine content is disclosed. A separation technique is utilized in which a gaseous product stream from a chlorine dioxide generator is scrubbed with an aqueous salt mixture containing an approximately stoichiometric quantity of sodium hydroxide. The scrubbing media is a controlled solution of sodium chlorate, sodium chloride and sodium hydroxide which is free of sodium carbonate. The process is not applicable for processing of sodium hypochlorite effluent streams. The sodium hydroxide reacts preferentially with the chlorine in the gas stream, yielding chlorine dioxide of high purity and converting the chlorine to sodium chlorate and sodium chloride which may then be recirculated to a chlorine dioxide generator.
Various processes are disclosed in the prior art for the destruction of an alkali metal hypochlorite, for instance, by reacting an alkali metal hypochlorite with an acid to produce chlorine, U.S. Pat. No. 4,404,179; the reaction of chlorine with hydrazine in U.S. Pat. No. 3,823,225; or the reaction of an alkali metal hypochlorite with urea, U.S. Pat. No. 4,508,697.
In U.S. Pat. No. 4,620,969 to Wilkinson, a process is disclosed for the production of chlorine by the electrolysis of an aqueous solution of sodium chloride. In part of this process, a gaseous stream containing chlorine and carbon dioxide are passed into a first reaction vessel and thence into a second reaction vessel and aqueous sodium hydroxide solution is charged to the first reaction vessel and aqueous sodium hydroxide is separately charged to the second reaction vessel. An aqueous solution containing sodium hypochlorite is removed from the first reaction vessel and an aqueous solution containing sodium carbonate is removed from the second reaction vessel.
In U.S. Pat. No. 4,129,484 to Larsson, a process is disclosed for the utilization of residual solutions obtained from a chlorine dioxide reactor in which sodium chlorate is reduced to chlorine dioxide in the presence of an acid. The residual solutions are converted to sodium chlorate by leading the residual solutions to an electrolytic cell having at the anode region of the cell an acid enriched fraction of the residual solution.
The process of the instant invention is particularly suited for the removal and reuse of the large volumes of alkali or alkaline earth metal hypochlorite produced subsequent to scrubbing the chlorine and carbon dioxide gases produced during the generation of chlorine dioxide by the reduction of an alkali or alkaline earth metal chlorate in the presence of an acid and methanol as a reducing agent. The carbon dioxide is vented to the atmosphere and the an alkali or alkaline earth metal hypochlorite is converted by the process of the instant invention to a dilute solution of an alkali or alkaline earth metal chlorate by reaction with chlorine gas or an acid. If desired, the alkali or alkaline earth metal chlorate can be recycled in a continuous process to an electrolytic cell for the production of an alkali or alkaline earth metal chlorate as a feed for a chlorine dioxide generator.